Observation And Simulation Of CG Lightning Activity Characteristics In The Regions With Tall Structures

This study focuses on the characteristics of cloud-to-ground(CG)lightning activities in isolated high structure areas and structure group areas,combining observations of the Tall-Object Lightning Observatory in Guangzhou(TOLOG)and simulations.The results will deepen the scientific understanding of the effect of the ground surface on the development of CG lightning of different polarities and reveal the differences in the distribution characteristics of+CG/-CG and their causes.Two-dimensional(2-D)simulation results on the+CG/-CG near the isolated structure show that the structure affects the spatial electric field distribution and,consequently,the development of lightning channels and their strike points.When the uniform distribution density of-CG is 1 km^-2,the-CG strike frequency of structures of100-600 m in height is 1.7,2.8,3.8,4.6,5.3,and 5.9,respectively,showing a positive correlation with its height.For the uniformly distributed+CG with a density of 1 km^-2,the+CG strike frequency is 0.34,0.55,0.76,0.96,1.16,and 1.35,which is much less than that of-CG.The-CG is not prone to strike the side of the structure,while the+CG side strike rate(the ratio of side strike frequency to all strike frequency on the structure)is 0,9%,17%,24%,31%,and 39%,respectively.Preliminary simulations prove the main reason for that is the positive leader is less likely to bifurcate.The higher the structure and the further the horizontal distance from the downward leader,the more likely it is to suffer a side strike.On the whole,the probability of top strike by+CG is still greater than side strike probability.Additionally,the structure may shift nearby-CG ground strike points toward itself.For taller structures,the deviation effect is more apparent.But the impact on the+CG is much weaker.When the density of ground strike points near the structure drops to less than 5%of the average,the corresponding distance is defined as the protection distance of the high structure.The structure of 100-600 m in height has the protection distance of 200 m,280 m,350 m,400 m,450 m,and480 m respectively for-CG,also positively related to its height.The protection distance for+CG is less than 100 m.The simulation and observation results of the high structure area in the Pearl River New Town show that the-CG strike probability on the structure is related to its height,shape,and distribution of nearby structures.The relative-CG strike probabilities(relative to the probability of the Canton Tower(the CT))of the eight tallest structures are 1,0.41,0.19,0.35,0.12,0.09,0.21,and 0.23,which has a good correlation with the observation data.The results are 99%,89%,81%,69%,61%,30%,19%,and 66%,respectively,of the probabilities when they exist in isolation.The effect of structures on+CG is not obvious,but the observed side strike rate by+CG is larger than by-CG.The CT can attract most of the downward-CG within a range of 1 km nearby,but the attraction to the+CG is not obvious.Upward lightning is prone to occur on supertall structures,especially the CT,which is basically triggered by the downward+CG within a range of tens of kilometers.A case of downward+CG related to the upward-CG from the high structure was observed by the TOLOG.The detailed analysis of optical and electromagnetic field data showed that the intracloud(IC)lightning process of upward-CG from high structure could also lead to downward+CG.